Hazard Potential Classification Systems for Dams, FEMA 333

IV. DISCUSSION
This Hazard Potential Classification System for Dams is based on the probable loss of human
life and the potential for economic losses, environmental damage, and/or disruption to lifelines
caused by failure of mis-operation of a dam or its appurtenances. This Hazard Potential
Classification System for Dams recognizes that the failure or mis-operation of any dam or waterretaining
structure, no matter how small, represents a potential danger to downstream life and
property. Whenever there is an uncontrolled release of stored water, there is always the
possibility, regardless of how unexpected, of someone being in the path of the discharge.
However, postulating every conceivable circumstance that might remotely place a person in the
potential inundation zone should not be the basis for determining the appropriate classification
level. This system considers improbable loss of life to exist where persons are only temporarily
in the potential inundation area.
The difference between the significant and high hazard potential classification levels is that a
high hazard potential dam includes the probable loss of human life. The failure of a dam that is
classified as a high hazard potential structure may or may not include adverse incremental
consequences that would otherwise justify a significant hazard potential classification.
The hazard potential classification assigned to a dam is based on consideration of the effects of a
failure or mis-operation during both normal and flood flow conditions. The classification
assigned should be based on the worst-case probable scenario of failure or mis-operation of the
dam, i.e., the assigned classification should be based on failure consequences that will result in
the assignment of the highest hazard potential classification of all probable failure and misoperation
scenarios. Each element of a project must be evaluated to determine the proper hazard
potential classification for the project. However, there is only one hazard potential classification
assigned to the entire project. Individual elements are not assigned separate classifications.
The probable scenarios considered should be reasonable, justifiable, and consistent with the
Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for
Dams (FEMA). For example, assuming reasonable breach parameters and a failure during
normal operating conditions (“sunny day” failure) may result in the released water being
confined to the river channel and no probable loss of human life, indicating a low hazard
potential classification. However, if the dam were assumed to fail in a similar manner during a
flood condition, and the result would be probable loss of human life (excluding the occasional
passer-by or recreationist) but minor economic losses, a high hazard potential classification
would be appropriate. Once a project is placed in the high hazard potential classification,
additional probable failure or mis-operation scenarios need only be considered if there is a need
to determine if they would likely induce higher adverse incremental impacts.
In most situations, the investigation of the impact of failure or mis-operation of a dam on
downstream human life, property damage, lifeline disruption, and environmental concerns is
sufficient to determine the appropriate hazard potential classification. However, if failure or
mis-operation of a dam contributes to failure of a downstream dam(s), the hazard potential
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